Electrohydraulic and pneumatic servovalves serve to provide a flow output which is linearly proportional to an electrical input for use in closed loop position control systems. These valves particularly require linear flow gain characteristics through that portion of the flow gain which is concerned with very small inputs and small resulting flows (the null-flow region) in order to maximize both the accuracy and stability of the position control loop and to minimize wasted energy in the form of internal leakage.
Typical electrohydraulic or pneumatic servovalves and proportional valves approach this null-flow problem by a process called flow grinding. In this process, a typical cylindrical spool intended for axial movement in a housing has its edges carefully ground in a process comprising measurement of the actual flow characteristics of each spool land; alternating with careful grinding of the edge with a precision grinder. This approach, if accomplished manually, requires highly skilled labor, precision machines, and substantial time. Automation of this process relieves the requirement for skilled labor and decreases the time, but requires a large investment in machinery. Another approach is to very carefully manufacture the spool and its mating housing or sleeve to attain the correct edge condition. This process requires holding tolerances better than 0.0001 inch on both parts consistently, which is expensive. Finally, the use of electronic compensation circuits to increase the electrical gain to compensate for low hydraulic gain in an overlapped valve results in a more linear flow characteristic. However, this still requires careful machining and processing and also requires that compensation circuits with various gains be available to allow some mismatch in the hydraulic gain.
Some rotary spool servovalve designs do exist in the prior art. Those known to the Applicant are: U.S. Pat. No. 3,007,494 to Herzl; U.S. Pat. No. 4,232,586 to Elser; U.S. Pat. No. 4,290,452 to Takahashi, et al.; U.S. Pat. No. 4,335,745 to Bouveret, et al.; U.S. Pat. No. Idogaki, et al.; U.S. Pat. No. 4,794,845 to Vick; U.S. Pat. No. 4,799,514 to Tanaka, et al.; U.S. Pat. No. 4,800,924 to Johnson; U.S. Pat. No. 4,848,402 to Elser, et al.; U.S. Pat. No. 4,858,650 to Devaud, et al.; U.S. Pat. No. 4,922,949 to Mizukusa, et al.; and U.S. Pat. No. 4,964,612 to Maggioni, et al., which will be discussed below.
To obtain a straight line flow vs. input curve throughout the operating range of the servovalve, most servovalve designs utilize rectangular metering slots which engage with spool lands which have corresponding straight cross sections. These rectangular metering slots require electrical discharge machining (EDM) to very precise tolerances, which is generally a slow and expensive machining method. Although some valves use round-hole ports to avoid this operation, the flow gain resulting from this method is inherently non-linear. In one case (Elrod, et al, U.S. Pat. No. 5,285,715) electronic compensation is used to overcome this problem.
The valve spool mechanization disclosed in this application uses a unique design and manufacturing process to eliminate the requirement for flow grinding of the servovalve spool, while using only simple and readily available machine shop processes to achieve the required lap condition. The design also has the advantage of using very simple part configurations which can be readily machined on standard machine tools. Finally, the round hole ports which are used in this invention require only standard drilling operations to machine, yet the configuration of the round pin in a slot of narrower width results in a linear flow gain curve over the entire valve stroke.